When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a radio frequency (RF) magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image (MRI) or produce a spectrum (MRS).
Radio frequency antennas or coils are used to produce the excitation field B1 and other RF magnetic fields in the subject being examined. Such coils are also used to receive the very weak NMR signals that are produced in the subject. Various types of coils are known, including large, whole-body RF coils which are built in to the MR system and produce a uniform RF field throughout a large region. Even though MR data may be acquired only from a small volume or slice in a subject placed in the MR system bore, the RF field from a whole-body RF coil permeates throughout the subject. The advantage of the whole-body coil is that the region of interest being imaged can be shifted to any location in the patient by changing the pulse sequence. Other known coils include various types of smaller “local” coils that are designed for specific clinical applications such as head imaging, knee imaging, wrist imaging, etc. Local coils may be either volume coils or surface coils. While local coils do limit the region that is subject to RF excitation fields during a scan, the shape and homogeneity of their fields is often not appropriate for a particular application.
The most common whole body coil found in commercial MRI systems is the so-called “birdcage” coil first disclosed in U.S. Pat. Nos. 4,692,705; 4,694,255; and 4,680,548. A birdcage coil has a pair of circular end rings which are bridged by a plurality (typically 8 to 24) of equi-spaced longitudinal straight segments. In a primary mode, currents in the straight segments are sinusoidally distributed which results in good B1 field uniformity across the axis of the coil. Birdcage coils are the structure of choice in horizontal field MRI systems because they produce a homogeneous magnetic field in the bore of the magnet and they have a high SNR which enables them to pick up the small NMR signals emanating from the examined subject.
An issue with all RF coils used to generate the B1 field is the level of electromagnetic energy which can be absorbed by a human subject during a scan. The FDA has imposed limits on such Specific Absorption Rate (SAR). Some pulse sequences employ RF excitation pulses of long duration which may expose the subject to excessive RF radiation. This is particularly problematic at high polarizing magnetic field strengths where SAR limits are easily exceeded. In these applications it is particularly desirable to limit the RF radiation as much as possible to the region of interest from which MR data is to be acquired.
There are a number of clinical applications where MR images are acquired at different Larmor frequencies. Hydrogen (1H) is the spin species of choice for most MR imaging applications, but other paramagnetic spin species such as phosphorus (31P), fluorine (19F), carbon (13C), sodium (23Na), helium (3He) and xenon (129Xe) are also of interest.